Some physical layer (PHY) standards in wireless communication system defines a structures data sequence that has a fixed duration for communication between a user equipment and a base station. The structures data sequence is referred to as frame. One frame includes a specific number of time resource units. A time resource unit may be called a subframe or a slot. One subframe/slot may be configured so as to include a plurality of OFDM symbols in the time domain. For example, one subframe may be constructed of 2 slots, each including 7 OFDM symbols. The number of subframes per frame, the number of slots per subframe, and the number of OFDM symbols per slot are determined according to the physical standard of a corresponding system.
FIG. 1 schematically illustrates three duplex modes used for bidirectional wireless communication.
UL/DL configurations in a frame depend on a duplex mode. The duplex mode means bidirectional communication between two devices, distinguished from simplex which means one-way communication. In duplex communications, transmissions on links in both directions may occur simultaneously (full-duplex) or at mutually exclusive times (half-duplex).
A full-duplex transceiver is used to separate two communication links, which are in opposite directions from each other, in a frequency domain. In other words, different carrier frequencies are used for the respective links, which is referred to as frequency division duplex (FDD). On the contrary, a half-duplex transceiver is used to separate two communication links, which are in opposite directions from each other, in a time domain. A duplex mode in which the same carrier frequency is used for the respective links is referred to as time division duplex (TDD). The half-duplex transceiver may use different carrier frequencies for the two links, which is referred to as half-duplex FDD (HD-FDD). In the HD-FDD mode, communications in opposite directions for specific devices are performed at different instants of time as well as at different carrier frequencies. Accordingly, the HD-FDD mode can be considered as a hybrid of FDD and TDD.
FDD adopted by most communication standards is very efficient for systems having wide frequency bands since it has a paired spectrum. In addition, FDD can dynamically change DL/UL configurations. On the other hand, TDD segments the same frequency band by time to process both DL and UL. While TDD is advantageous to systems which mostly use traffic such as Voice over Internet Protocol (VoIP), it has disadvantages that a geographical area which can be covered by each communication device is smaller than that of FDD and a guard period is needed between a DL period and an UL period due to restrictions on Round Trip Time (RTT). However, TDD can use unpaired spectrum in case of insufficient frequency spectrum. FDD requires a transmitter and a receiver to include a duplexer, a filter, which is capable of separating UL and DL signals transmitted at different carrier frequencies from each other with high accuracy to transmit and receive signals simultaneously. The duplexer causes a certain degree of signal attenuation. This attenuation is generated in a low-noise amplifier on a signal path in the receiver, and thus it directly affects the noise level of the receiver to deteriorate the sensitivity of the receiver. In the case of the transmitter, the duplexer is located behind a power amplification stage on a signal path and requires a high-power amplifier for the power amplification stage in order to overcome or endure the signal attenuation. On the contrary, TDD does not require a transceiver to transmit and receive signals simultaneously, and thus the transceiver can be implemented without a duplexer, simplifying their circuit layouts.
However, a TDD system is allowed to use only one DL/UL frame configuration even though it defines a plurality of DL/UL frame configurations at the system level in order to maintain inter-cell interference between DLs and/or ULs uniform at the network level higher than the system level to reduce system complexity. In other words, while most TDD systems define various DL/UL frame configurations, each BS is restricted from freely configuring DL/UL frames. In 3GPP LTE system, for example, one network cannot have different DL/UL subframe configurations, and thus only one DL/UL subframe configuration is used on a network in which neighboring cells are organically linked to each other. A TDD system in which BSs have the same DL/UL subframe/frame configuration in the same network, as described above, is called a symmetric TDD system.